Semiconductor laser diodes are generally known in the art. Typically, semiconductor laser diodes include parallel facets that are formed when semiconductor laser diodes are formed by cleaving a semiconductor crystal along the crystal's natural cleavage planes. The facets help confine light that is emitted by a p-n junction located within the semiconductor laser diode by reflecting back into the semiconductor body a fraction of the light that otherwise would exit the semiconductor body. This reflection of the emitted light promotes a condition where the reflected light oscillates within the semiconductor body. This p-n junction is formed by p-type and n-type doped layers grown on a substrate. P-type and n-type contacts are formed on the surface of these layers.
Because of their numerous advantages, including low cost, small size, high mechanical stability, potential for substantial output power, and very good efficiency (often near 50% for pulsed junction semiconductor laser diodes), semiconductor laser diodes have the potential to replace large inefficient and expensive laser systems in many industrial, scientific, medical and military applications. Semiconductor laser technology also presents one of the most efficient and adaptable methods of generating coherent laser radiation at different wavelengths. By varying the type of semiconductor alloy from which the semiconductor lasers is made, the semiconductor laser diode can produce radiation at a range of wavelengths.
Semiconductor laser diodes can be used as the optical pumping source for fiber-optic networks and communication systems. For example, wavelength division multiplexing (WDM) fiber optic networks use 980-nm pump lasers to amplify signals transmitted through the fiber optics systems simultaneously at different wavelengths. The increased demands on and popularity of WDM fiber optic networks have increased the demands on pump lasers, and increased output power from 980-nm lasers is crucial for high-speed communications systems.
Semiconductor laser diodes can have thermally related issues that limit their ability to provide increased output power. The thermal issues are related to the large heat dissipation of the laser diode per unit area from the laser diodes, which causes elevated junction temperatures and stresses. The optical output of a laser diode declines as it heats up; thus, increases in junction temperature tend to decrease the efficiency and service life of a laser diode. Junction temperature also affects the emitted wavelength of a laser diode. Maintaining a constant junction temperature is important in maintaining a given output wavelength.
When the laser diode is operated at high output power, the temperature increase at the facet due to nonradiative recombination can be large. If the temperature at the facet exceeds the melting point of the semiconductor material used to form the laser, rapid destruction of the facet occurs, which inhibits proper laser diode operation. The increase of junction temperature near the facet may cause catastrophic optical damage (COD), which is permanent damage to the facet. This destruction of the facet results in failure of the device.
Heat sinks often are used with laser diodes to help with heat dissipation, but heat sinks are limited in the amount of power they can dissipate. In addition, present laser diode designs often do not effectively transmit enough of the heat to the heat sink. Increasing the size of the laser diode itself can increase the surface area through which heat can be dissipated, but this method tends to be limited by the size of the chip.